For the first time, the complete structure of the lipid A from the lipopolysaccharide of an Agrobacterium species is here reported. In particular, the structure of the lipid A from A. tumefaciens strain C58, a soil pathogen bacterium strictly related to Rhizobiaceae, was determined. The structural study, carried out by chemical analysis, mass spectrometry, and nuclear magnetic resonance spectroscopy, revealed that lipid A fraction consisted of a mixture of species all sharing the bis-phosphorylated glucosamine disaccharide backbone that could be designated in two main structural motifs, according to the acylation pattern. The main species was a penta-acylated lipid A bearing two unsubstituted 14:0 (3-OH) fatty acids in ester linkage and two 16:0 (3-OH) in amide linkage; the one on GlcN II was O-acylated by a long chain fatty acid, 28:0 (27-OH). This in turn was esterified by a 3-hydroxy-butyroyl residue at its hydroxy group. The second species, in lesser amounts, was identified as a tetra-acylated lipid A and lacked the 14:0 (3-OH) residue on GlcN I. Other species deriving from these two lacked a phosphate group or 3-hydroxy-butyroyl residue or otherwise carried a 26:0 (25-OH) as long chain fatty acid. The lipid A structure of phytopathogen A. tumefaciens strain C58 presents deep structural analogies with lipid A of symbiotic Rhizobium, and the hypothesis is advanced that it can be a strategy of the bacterium to escape or attenuate the plant response.